The conventional writing pen is well-known in the art. One of the primary characteristics of the established design of a writing pen is that the pen tip is visible to the user. This allows the user to visually connect his writings to each other. Recently, ink jet print technology has been incorporated into a pen to form a hand-held ink jet pen. Ink jet printing is a conventional technique by which printing is accomplished without contact between the print head and a substrate or medium, on which the desired print characters are deposited. Such printing is accomplished by ejecting ink from the ink jet print head of the ink jet pen via numerous methods which employ, for example, pressurized nozzles, electrostatic fields, piezo-electric elements and/or heaters for driving liquid to vapor-phase change droplet formation. Some of the hand-held ink jet pens of the prior art have employed a measurement means for measuring, without physical contact, the distance between the print head and the substrate. The measurement means is typically connected to a processor unit which is adapted to cause the ink jet system to be activated when the measurement means determines that the distance between the ink jet print head and the substrate is less than a predetermined maximum value and simultaneously a movement detector detects movement of the ink jet pen. However, such sensors require additional space that can depart from the conventional pen shape that a user has been so comfortable with over the years. As such, space is limited and places a constraint on the number of electrical sensors and connections that can be placed inside the physical constraints of the ink jet pen.
In the case of a traditional writing pen, line-width is a primary descriptor by which the customer makes his choice. Line-width is typically specified either directly in millimeters or by such adjectives as “bold”, “medium”, “fine”, or “extra-fine”, each with a specified meaning within the industry. Line-widths of 0.200, 0.300 and 0.500 millimeters are industry standards; although such descriptions apply directly only for a particular ink and paper combination and a particular pen tip speed. As such, when designing a print head, some of the technical challenges include determining the optimum number of heaters and nozzles, optimal spacial configurations and corresponding optimal spot size so as to achieve a specified line-width with a minimum of variation.
Line-width variation can come from multiple sources. These sources include: 1) variations in surface and absorption properties of the print media. These typically occur in media from different sources or even from a single unit from the same source; 2) variations in environment, particularly in temperature and humidity. These cause variations in the moisture content of the print medium and thereby lead to variations in ink absorptive properties; 3) variations in drop mass and jet velocity caused by variations in reservoir back pressure, heater conditions, etc.; and 4) variation in the user's manner of holding and moving the pen.
The first three sources are well-known to those skilled in the art of traditional ink jet technology. The fourth listed source of variation (the user manner of holding and moving the pen) is unique to the hand-held ink jet writing pen. As such, there is a need for a hand-held ink jet pen having a print head configured to minimize variations in line-width due to orientation of the ink jet pen. Accordingly, improved ink jet pens are desired.